High molecular weight linear polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319, and in Pengilly, U.S. Pat. No. 3,047,539, incorporated herein by reference. These patents disclose that the polyesters are particularly advantageous as film and fiber formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection-moldable compositions. Poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Workpieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Recently, block copolyesters, wherein the major portion of the repeating units are poly(1,4-butylene terephthalate) blocks, have been found to have enhanced impact resistance. See copending application U.S. Ser. No. 752,325, filed Dec. 20, 1976 which is now abandoned.
Enhanced impact resistance has also been reported when poly(1,4-butylene terephthalate) and block copolyesters have been combined with impact modifiers comprising resins, e.g., block copolymers of vinyl aromatics and dienes, in combination with aromatic polycarbonate resins and a mineral filler. Flame retardant such compositions are also known. See copending application, U.S. Ser. No. 966,864, filed Dec. 6, 1978 which is now abandoned.
Stable polyblends of poly(1,4-butylene terephthalate) and poly(ethylene terephthalate) can be molded into useful unfilled and filled articles. See Fox and Wambach, U.S. Pat. No. 3,953,394.
In general, poly(1,4-butylene terephthalate) resins are known to be suitable for combination with glass fiber reinforcement and/or flame retardant agents.
U.S. Pat. No. 3,937,757 teaches that the electrical arc tracking resistance of unfilled poly(1,4-butylene terephthalate) compositions can be improved by the addition of from 5 to 50% by weight of a polyolefin or ethylene copolymer, containing at least 50% by weight ethylene units thereto.
Copending application U.S. Ser. No. 972,888, filed Dec. 26, 1978, discloses that the arc track resistance of poly(1,4-butylene terephthalate) compositions can be improved by forming a combination of the polyester resin, a copolymer of ethylene and vinyl acetate and a filler, and then post-reacting them.
The above-mentioned patents and applications are incorporated herein by reference.
Unfortunately, however, the attempted use of flame retardant agents in such compositions causes a lowering of arc track resistance, and this is further lowered if such flame retardant modifications are further compounded with blends of a polyolefin or olefin copolymer and an aromatic polycarbonate, or poly(ethylene terephthalate), or glass fiber reinforcement.
It has now been discovered that if a mineral filler is included in such flame retardant modifications, in a controlled amount, there is a surprising improvement in arc track resistance (as measured by ASTM D-495 test) as compared with the corresponding non-mineral filled flame retardant blends. This discovery permits the use of such polyester compositions in such fields as parts for automotive electrical systems, and the like.